Time-of-flight (TOF) is a technique used to determine the distance to an object or objects in a three-dimensional scene. Such techniques can be used to construct three-dimensional representations of an object or a three-dimensional scene. TOF-based optoelectronic modules frequently employ a modulated light source, a series of optical elements, and a demodulation pixel or demodulation pixel array. Modulated light projected from a modulated light source onto an object in a scene may be reflected to an imaging assembly and focused onto a demodulation pixel. The demodulation pixel and supporting circuitry, may detect a phase shift in the reflected light, wherein the phase shift may be further correlated to the distance the light traveled; specifically, the detected phase shift is the phase shift in the modulated light that transpires when the modulated light travels from the light source to the object and is reflected back to the TOF module. Accordingly, the phase shift (phase delay) is proportional to the transit time as expressed below:
      t    tof    =      -                  ϕ        tof                    2        ⁢        π        ⁢                                  ⁢                  f          mod                    where ttof is the time-of-flight, ϕtof is the phase shift of the modulated light signal, and the respective modulation frequency is fmod. The distance to the object (Rtof) can then be calculated according to the following:
      R    tof    =                    t        tof            ·      c        2  where c is the speed of light. Alternatively, the round trip time can be directly measured in order to calculate the distance to the object.
Other techniques may be employed for determining distances to objects in a scene such as triangulation. Triangulation-based optoelectronic modules often use a light source, a series of optical elements, and a pixel array. As above, light projected from the light source and reflected by an object in a scene may be focused onto the pixel array via the optical elements. Distance to the object then is determined via a standard triangulation technique where distance is determined from the focal length (i.e., on-axis focal length) of the series of optical elements, the position of the pixel on which the reflected light is focused (e.g., as a spot), and the baseline distance between the on-axis focal length and the illumination source. (Rtri is the distance information obtained by the triangulation measurement; f is the on-axis focal length; b is baseline; xpix is the location of the pixel on which the reflected light is focused (e.g., a spot of light); α is the angle between emitted signal and measurement axis
      R    tri    =            f      ·      b                      x        pix            +              f        ⁢                                  ⁢                  tan          ⁡                      (            α            )                              Further, if α=0 the formula simplifies to Rtri=(f×b)/xpix). TOF can yield superior distance data for some applications while triangulation may be better suited to other applications.
Small distance-measurement opto-electronic modules can be used in a wide range of applications. For example, they can be integrated into smart phones or other small electronic devices such as handheld or other Internet-enabled (or other network-enabled) personal computing devices, including personal computers, e-books, kiosks, tablets and media players. In the context of smart phones, for example, the small distance-measurement modules can be used to determine the distance between a display screen and the user's hand for gesture recognition. In some instances, the distance measurement modules can be used to determine the distance from the device to a user's body (e.g., face or ear) to determine when the user likely is not looking at the display such that the brightness level for the display screen can be reduced.
An important characteristic of distance-measurement modules for some applications is the required refresh rate, in other words, the frequency at which the module (or the host device) is supposed to measure distance. One factor impacting the refresh rate is the maximum latency, which reflects how quickly the module (or host device) needs to detect movement of an external object. The distance measurement refresh rate also depends on the maximum acceptable error at any given time.
Turning on the distance-measurement module's light source tends to consume a relatively large amount of energy. Thus, in battery-operated devices, a high refresh rate can cause the battery to discharge more quickly.